I faced this problem. I dont know how to formulate this as min cost flow problem:

A rental company uses cars which it leases from manufacturers. The company has a requirement of cars for the next 6 months as follows: Mar: 43 Apr: 41 May: 44 June: 39 July: 42 Aug: 45

The company can lease cars for the following costs and lengths of time: a 3-month lease for $1700, a 4-month lease for $2200, a 5-month lease for $2600. The company can undertake a lease beginning in any month. On March 1 the company has 20 cars on lease, all of which go off lease at the end of April.Formulate the problem of determining the most economical leasing policy as a mincost flow problem.